1. Field of the Invention
The present invention generally relates to a Hybrid Automatic Repeat reQuest (HARQ) method in a mobile communication system. More particularly, the present invention relates to a method for performing HARQ according to multiplexing and resource allocation in an Orthogonal Frequency Division Multiple Access (OFDMA) system.
2. Description of the Related Art
In general, wireless communication systems are designed to provide communication services to users, irrespective of location. The wireless communication systems have been developed to accommodate multiple users by multiple access schemes. A major multiple access scheme is Code Division Multiple Access (CDMA). CDMA has evolved from voice communication to relatively high-speed data transmission. A driving force behind the CDMA development is a drastic technological development along with user demands for high-speed data transmission. Owing to the technological development, most of 3rd Generation (3G) mobile communication systems have been standardized and have also been commercialized.
Due to limited CDMA resources, however, there is a limit on increasing data rate. Nonetheless, user demand for higher data rates continues to increase. In this context, many studies and attempts have been made to improve high-speed data transmission in the wireless communication field.
One of the studies is OFDMA. OFDMA is a technology in which a plurality of channels are configured using orthogonal frequencies and at least one of the channels is allocated to each user, for data transmission.
A brief overview of OFDMA communications will be presented below.
In OFDMA, communication is made by allocating a downlink subchannel and an uplink subchannel. That is, a downlink period and an uplink period are separated within a given time period and the downlink and uplink subchannels are allocated to a user in the downlink and uplink periods, respectively. An OFDMA cellular mobile communication system uses an available frequency in two ways. The frequency utilization is based on a frequency reuse factor.
One of the frequency utilization methods, which is more popular than the other, is that a frequency reuse factor is larger than 1, such as 3 or 7.
Typically, OFDMA physical channels support two types of resource channel structures: Localized Resource Channel (LRCH) and Distributed Resource Channel (DRCH).
In the LRCH structure, a user is assigned a set of contiguous subcarriers at the same positions through a few OFDMA symbols. The set therefore defines time-frequency resources with regularly spaced contiguous subcarriers on contiguous OFDMA symbols. With the LRCH structure, a time-frequency region at a good channel status is allocated to a Mobile Station (MS) in a good channel environment.
With the DRCH structure, a user is allocated a set of subcarriers scattered across a particular time-frequency region. Hence, the DRCH is allocated to an MS that intends to achieve frequency diversity.
FIG. 1 illustrates the LRCH structure and the DRCH structure in the OFDMA system. Referring to FIG. 1, a total available frequency band has a plurality of LRCHs or DRCHs. The total number of the available channels varies depending on certain situations. In the LRCH structure, the entire frequency band is divided into a plurality of subbands along the frequency axis, each being defined as an LRCH for use in subband scheduling.
FIG. 2 illustrates an exemplary DRCH structure in the OFDMA system. The horizontal axis represents time, and the vertical axis represents frequency. Referring to FIG. 2, a minimum time unit is an OFDMA symbol and seven OFDMA symbols form one minimum transmission unit (e.g. slot) along the time axis. A minimum frequency unit is a subcarrier. One DRCH is formed with subcarriers regularly spaced by 8 subcarriers. Different DRCHs are marked differently. In the case illustrated in FIG. 2, each DRCH is repeated once every 8 subcarriers. This is called a repetition period, N. In FIG. 2, N=8. Each DRCH starts at a different subcarrier position in every OFDMA symbol. The starting subcarrier position is represented by an offset. DRCH 1 has an offset of 0 in a first OFDMA symbol, an offset of 3 in a second OFDMA symbol, and an offset of 6 in a third OFDM symbol. In this manner, DRCH 1 can be defined by an offset sequence of {0, 3, 6, 1, 5, 2, 7} on contiguous OFDMA symbols.
As described above, the total frequency band may have LRCHs or DRCHs during one slot. The DRCH and LRCH structures can both be used in one system. There are two resource channel Multiplexing (MUX) modes, MUX mode 1 and MUX mode 2 for the channel assignments, which are described with reference to FIGS. 3 and 4.
FIG. 3 illustrates MUX mode 1. Referring to FIG. 3, DRCHs are defined across a total frequency band, and LRCHs are defined as time-frequency resources in preset subbands. An LRCH is configured with the remaining time-frequency resources except for time-frequency resources (one subcarrier per OFDM symbol) allocated to a DRCH in a subband mapped to the LRCH.
As more DRCHs are sent, the number of time-frequency resources on which the DRCHs are punctured onto LRCHs increases. The remaining DRCH resources can be utilized for the LRCHs. Consequently, resources are efficiently utilized.
FIG. 4 illustrates MUX mode 2. Referring to FIG. 4, the DRCH structure and the LRCH structure are only used on different zones. An LRCH zone is first defined, and the remaining zone is used as a DRCH zone. In MUX mode 2, the same number of LRCHs are configured along the total frequency band and then DRCHs are configured in the remaining zone. Due to the strict distinction between the DRCH zone and the LRCH zone, even though more users are allocated DRCHs, fortunately the amount of resources allocated to LRCH users is not changed. However, since DRCHs should be filled in the remaining time-frequency resources other than those for the LRCHs, resources are wasted when there are a small number of DRCH users or no DRCH users.